Louis and other Picsters:

Generally all piezoelectric transducers have many response peaks above
and below the designed "fundamental" operating frequency. Depending on
how it is connected in the circuit non-fundamental responses may be the
highest. That is, coupled with the amplifier bandpass characteristics,
the transducer may in fact produce a very high lower frequency response
as you observed.

From graduate research work I and my colleagues did at Stanford on
medical ultrasound (both CW and pulsed Doppler) in the early 70's we
found:
1. Impedance match transducer to circuit and transmission medium (be it
air or water) for highest sensitivity.
2. Dampen high Q transducers with mechanical or electronic loading to
improve spacial resolution in pulsed mode applications.
3. Limit out of band responses (measure the bandpass of loaded
transducer, impedance matching network and preamp) by adding low pass or
high elements. (It takes real skill in the front end to get just one
response peak with high sensitivity.)
4. Use lowest noise preamp possible with gain of 10x to set noise floor
of system. (You use a lower first stage gain but that puts lower noise
requirements on the following stages.)
5. Isolate TX and RX transducers electrically and acoustically if a two
transducer design or use active damping (and release) and switched
preamp gain for pulsed single transducer designs.
6. Isolate the transducers from the ambient environment except for the
desired acoustic viewing 'window' to the target.

Hope this helps. Have fun.

- Wayne Foletta
BMI - Santa Clara

> ----------
> From:         Louis Marquette[SMTP:lpm2@LEHIGH.EDU]
> Sent:         Wednesday, November 19, 1997 9:11 PM
> To:   PICLIST@MITVMA.MIT.EDU
> Subject:      Re: Serial Sonar Unit (SSU)
>
> > >By placing the transducers inside of pipes or cylinders lined with
> > some
> > > sound-absorbing substance (like velvet or some other cloth) we
> > should be
> > able
> > > to get good directivity, and with different resistor values in the
> > op-amp
> > > circuits, we should be able to tweak the gain to accomodate the
> > longer
> > > distances.  I have been planning on 8 bits of granularity in the
> > measurements,
> > indoors, presumably due to reflections. The other guy who was the
> main
> >
> > designer of the project (who is also on this list, Louis Marquette),
> > has
> > since tried placing the transducers in cylinders and I think that he
> > had
> > good success.
> >
>
> When I tried to resolve the return signal detected,  I seemed to
> always
> have some signal being detected. Looking back, it seems that i used a
> 3
> transister amplifier, and i tried all sorts of gain configurations. I
> think that just using 3 transistors rather than on  with alot of gain,
> inserted some noise at each stage of the amp. you might want to try a
> comparator to compare the 2 terminals of the transducer, that may work
> well, but back to the tubes, I wasnt sure if it was my amplifier
> introducing the noise or of it was amplifing small signals made my the
> transducer. for all of you out there who say that the transducer has a
> narrow bacdwith and so does not need filtering, it is a filter, well,
> true enough, but if you tap one, even lightly, it will generate a
> small
> signal, i dont know if this is because tapping it causes it to
> resonate
> at 40khz or if the mechanical stress is transfered to the piezo
> element
> and converted to a voltage, either way it is more or less the same
> effect. It will also create a small voltage if ait rushes past it or
> it
> 'hears' a loud noise, even if the noise is MUCH less than 40k. I
> placed
> a little conductive foam used for static sensitive ic's(hey, its
> handy!)
> around the transducers outer edges, used that as an insulator of sorts
> from a 5cm long tude i put it in, transducer diameter was abt 1 cm,
> the
> tubes was abt 2cm. then i insulated the tube from shock from the
> surcace
> it was mounted on with mre foam. it seemed to work better, but lots of
> false detection problems still.oh well thats life! if anyone has any
> really good breakthroughd, please let me know, as i would like to
> continue but grew frustrated. well, my osciliscope died too, and i'm
> not
> done with my homemade pc based on yet.
> Louis Marquette
>
>
> > I have also seen in several places recommendations that a specific
> > pattern
> > of pulses be used rather than a striaght 40KHz burst. This allows
> you
> > to
> > throw out noisy readings because the return pulses will not fit the
> > expected pattern. I imagine that the pattern must be carefully
> > designed so
> > that it is still recognizable after being passed through the
> > transducers,
> > which act effectively as a narrow 40KHz band pass filter.
> >
>